"When I do install it, would I be able to use a Windows 95 or 98 boot disk "
In most cases, yes, you can boot the mboard itself with a bootable floppy that has one of those earlier op systems on it , but some recent mboards have no main chipset drivers for anything previous to Windows 2000, or previous to XP, so on those mboards the older op systems may not recognize the chipset properly and may not work.
However, the Win 95 boot disk might be relatively useless - versions of 95 previous to OSR2, the vast majority of the versions out there, cannot deal with any more than a 8.4gb hard drive, and only then if it has 4 2.1g partitions (those are the limitations of FAT - FAT 16 - partitions).
"can I still have the option of having the partitions formatted as NTFS?"
Versions of Windows previous to 2000 and the later versions of NT (9x to ME) cannot natively see a NTFS partition.
If you want a partition on the drive to be seen by 98, 98SE, and ME, it must be FAT32 or FAT. If you want a partition to be seen by 95, versions previous to OSR2 can only recognize FAT partitions not including FAT32, and the FAT32 support in versions OSR2 or later has limitations the FAT32 in 98 and later does not have.
2000 and up will not allow you to partition a drive partition larger than 32gb as seen in Windows to anything but NTFS in the operating system itself, or when you run it's Setup, but you can partition a drive partition FAT32 if it is larger than 32gb with third party programs, such as the free hard drive preparation programs available from the hard disk manufacturer's web sites, or sometimes included with a retail packaged hard drive, and 98 and up will see the full size of the partition properly, although if it is larger than 32gb you must install a patch for 98 and 98SE so the drive is recognized properly by everything in Windows. I think 98 to ME have a 128gb max size a partition can be made in the operating system itself, but they will recognize a partition larger than that if it was made by a third party program. FAT32 has the limitation any one file can be no larger than 4gb. 98 and 98SE must have an updated Fdisk installed in order to partition a drive partition larger than 64gb as seen in Windows - the Windows CD does not support that either, and if you use a made in Windows 98 or 98SE Startup floppy disk, it copies the needed files for it off of the CD, so you have to replace the Fdisk on the floppy with the newer version.
You don't need larger partitions for ME and previous. E.g. I've had 98SE on a 7.8gb partition for years.
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All the partitions on the hard drives must be partitioned and formatted if you want to be able to put data on them.
You cannot format a blank drive or blank partition without it having been partitioned first. In XP, in Disk Management, it may seem you are just formatting a blank partition, but that is a illusion - it has to be partitioned before it can be formatted.
The drive can be blank when you first connect it - when you run Setup from the CD, one of the first things it does is check for existing partitions on your connected hard drives - if there is one or more, you are asked whether you want to install Windows on it or one of them, and if you do, you can delete the existing data on it by installing Windows on that partition, which by default for 2000 and up deletes all the data on that partition,
- if there are no partitions, it offers to partition at least one partition , you can choose a partitioning type other than NTFS if it is smaller than 32gb, then it is formatted to suit. The default is for it to make a single partition on a drive, but you can specify a smaller size at that point. You can backtrack in Setup once one partition has been partitioned and formatted, and partition and format other partitions, but the default is Setup is only concerned about you having a partition Windows itself can be installed on. You can partition and format a blank drive or a blank drive partition at any time by using Disk Management in Control Panel. The blank partition or the entire blank drive is not seen in My Computer or Windows Explorer until it has been partitioned and formatted to some type the operating system recognizes.
" I don't plan to create multiple partitions, at least not yet."
You can't change how many partitions you have on a drive in the operating system itself, if the partition(s) it has fill(s) the drive, without losing the data contents of the existing partition(s). You can, however, use third party programs generically called partition manipulation programs, such as Partition Magic 8.x, that can re-size existing partitions smaller without you losing the data on existing partitions, if the data does not fill the partition, and make one or more new partition(s) in the freed up (unallocated) space on the drive (but you are advised to back up important data before you do that).
It is DUMB in my opinion to make a single partition on a hard drive, especially if it is the only one, and/or if it is huge.
If you ever have to re-install Windows from scratch at any point, you lose the entire contents of the partition Windows is installed on, unless you backup what you need to save elsewhere beforehand. If you have at least two partitions on the computer and the other partition(s) is(are) not full of data, if nothing else you have a place on the computer you can store data you don't want to lose when you re-install Windows from scratch, or a place you can backup data on the Windows partition to if you haven't already done so.
You DO NOT have to install most programs and other data that did not come with Windows on the same partition Windows is on. In most cases you can choose other than the default Express or Standard whatever installation choice, and choose to, or change the install location to, install it on another partition. If it is a program or program package, it will install a little bit on the Windows partition, and the vast majority of the program(s) on the other partition.
If you have to re-install Windows from scratch, you will have to re-install programs that were on the other partitions too, but any files and personal settings added after the program was installed on the other partition will still be there on the other partition.
"If at some point I do use this new hd to install the OS, is there a maximum size the partition where the OS is located can be?"
There is a partition size limit, but it doesn't specifically apply to the partition Windows is on, and it is of no concern for NTFS in 2000 and later because there are not now, and may never be, hard drives large enough such that that limit is reached. The limit for a FAT32 partition is smaller, but that too isn't a concern.
