Vgr seems to have provided you with most of the answers you requested.
As far as quick and dirty methods of boosting enemy difficulty - increasing enemy level is a must. When I modded psx ff7, Proud Clod hadn't yet integrated enemy stat modification - I used the now obsolete Hojo tool. I don't know how PrC handles changing enemy stats, but I would just go from scene to scene and increase enemy levels, and pretty much each of their stats. For the enemies from New Game -> Junon or so, you can safely double all their levels (in fact, you may want to triple or quadruple the levels of enemies within Midgar). The amount by which you decide to increase enemy HP will depend on the changes you make using Wallmarket. If your team can cause MORE damage earlier in the game as a result of your changes, I'd say at least DOUBLE enemy HP. If you make Kernel changes that DECREASE your teams damage output, perhaps you'd be safe leaving HP alone and just boosting enemy level and stats. Dexterity can be a pretty interesting stat to play with for enemies. Increasing it too much will result in enemies attacking you multiple times per ally turn. But leaving it unmodified and increasing the other stats could work fairly well. If this is your first time trying to make a gameplay mod of FF7, I suggest that you don't get too far ahead of yourself or too ambitious - mostly because you can easily multiply the amount of time required to complete your mod by adding two or three great new ideas you have. Also, you WILL encounter glitches if you HEAVILY mod the kernel and scene files.
For some hex editing (or gameshark codes) that can add different elements to the game and eliminate the need to make certain file modifications, I suggest you visit Gamehacking.org
http://gamehacking.org/?sys=psx&q=F&p=4 Good luck.