How would I be able to ensure that I am programming within the limits of the original hardware as to save me from having to back track and fix something later on and allow it to be playable on actual hardware?

I know the answer to the above question is to test the hack on real hardware (I personally can’t however I’m sure I can find somebody at the sonic forum) but are their certain guidelines I can follow to make sure I don’t run into any problems. Thanks!

Also I understand that real game cartridges have limitations such as space for example, but since the emulators attempt to accurately emulate the hardware only, the cartridge is a separate device, so the size of the rom shouldn’t really matter right? (or at least I don’t think it should matter). So as long as I program within the hardware (console) specifications I shouldn’t have an issue even if I create an extremely large rom hack (one that is larger than a normal genesis game found on a cartridge) Please correct me if I’m wrong or shed some light (also if this doesn’t make sense please let me know I can elaborate). Thanks

Make sure you read the official or known accurate technical documents on whatever features you're using beforehand. This can help prevent any problems. But yes, ultimately, you'll need to have it tested to really know you did something wrong and it needs to be fixed.

I'd stick to the largest ROM size that will load in the copier of those that are testing it. Easy answer there. If it's larger than that, nobody can test it unless they actually made a cart(especially a uncommonly large one) which I don't think many will do for you once or regularly for testing during development.

Sonic CD wasn't included either...  Also, it was available in the US as well (I own it).

More details.

Sonic Jam was fun I liked the world map idea

The hack I'm trying to make (just in the planning stages right now) would take the principle idea's that Cinossu used in his Sonic The Hedgehog Extended Edition hack and expand on them.

I really like the warp ring system that Cinossu uses in his hack among other things. Once I played the hack I thought of all kinds of things I could do to improve it. For one I could implement more characters, and include  more games, and even hide bonus material through out the game.

This is kind of rough interpretation but my hack would be a Metroid meets Sonic type game if that makes any sense.