i know its possible to run high sierra on a 2008 but would it be possible to just put a hard drive in that already has high sierra on it in? i have a hard drive with high sierra on it and some other files from my 2011 mbp 15" that died and its a larger drive too so it would work much better for me
An SSD-replacement for a spinning-drive is kind of mandatory to run (patched)Sierra/HighSierra/Mojave.
You can also install patched_Mojave, if you should prefer dark-mode.
As for Your pre-installed HS-drive: I'd recommend a clean install of a patched_HS/Mojave and migrate Your data.
A 500GB SSD would be a good choice for running a multi-boot-system.
Partitioning might look like this:
P#1.Patched_HS/Mojave (120G)
P#2.Data
P#3.SnowLeopard (for faster Speed and Rosetta-support) (60-80GB)
P#4.small Partition to hold the patched_HS/Mojave-Installer (16GB)
It's important to keep the order, if you want to delete/reintegrate partitions: deleting P#4 will add it to P#3, deleting P#3 will add it to P#2. But you cannot delete P#2 to make it part of P#3 etc.
If you want to keep compatibility use HFS+ file-systeme for all partitions.
@dosdude1's patched macOS versions allow to keep HFS+ file-systems even for HighSierra and Mojave.
To get started prepare the SSD via external USB-connection (get a 10$ external USB3-case - it can hold the preexisting spinning-drive for backup-purpose later) and create the above 4 partitions.
P#1 + P#2 are basic and meant for the main macOS and your data. P#3 is the "add-on" additional SnowLeopard-OSX (that might be deleted and added to the data partition) and P#4 is the backup/recovery patched installer if something needs to be re-patched.
Run dosdude's patcher and install the patched_macOS-installer on P#4, so you'll always have a fast installer and a recovery on board.
If you already have a running SnowLeopard-system use CCC or SuperDuper to clone the system to P#3 (which should be partitioned the size, your actual SL-system has - but move all your data, that should be available on both SL and HS/Mojave to the P#2 data-partition first to free space on the SL-partition.
If you want a fresh install of SnowLeopard, skip the previous cloning-step. You may do that later. Just prepare that 60-80GB partition, which should be big enough. (Remember, that you cannot increase P#3 unless you delete and add P#4)
Install the SSD (only patched macOS-installer on P#4 is necessary) into your 2008MBP.
Boot from P#4 and install patched HS/Mojave onto P#1. Keep HFS+ file system for P#1 (if you choose APFS you cannot access P#1 from your SnowLeopard-system)
After finishing patched_HS/Mojave installation and previous cloning of SL to P#3 your DualBoot-system is ready.
(if You skipped SL-Cloning You may now install SL onto P#3.
I've done this on both early and late 2008 MBP.
There should be at least 4GB RAM available even to run patched_Mojave.
Early2008MBP could be upgraded to 6GB RAM, the late2008MBP could be upgraded to 8GB of RAM, but make sure, if it's a late2008 model to have the last firmware update (MacBook Pro EFI-firmware-update 2.8), otherwise it won't be able to access the full 8GB RAM.
I've also tried OCLP Monterey and Ventura on a late2008 2,9GHz 15"MBP. Both need about 80GB for system and my basic software configuration.
OCLP_Monterey does work with some limitations (slower speed compared to Mojave, no Apple.Maps, no Apple-TV.App)
OCLP_Ventura refused to install.
So if you like to add OCLP_Monterey experience to your setting, You might add an 80-100GB partition between P#2 and P#4 and get the most maxed-out MultiBoot system.
@ihatemy2011macbookpro which 2008MBP-model do You have the early2008(silver keyboard) or late2008(black keyboard)?