Where can I find a list of guidelines or rules to build a plane that is legal for IMAC comp.

If you'r going to be in Basic Class you could fly an ugly stick.
In short you can fly anything in Basic Class.

Without actually looking up the rules (which are on the AMA website with a heading of Scale Aerobatics), the general rule for classes above Basic (Sportsman and up) is that it must be a model of a full-scale aircraft that does fly or has flown in full-scale aerobatic competition, or is capable of doing so, and the model must be true to scale within 10%--which gives you the builder or a kit/ARF manufacturer some leeway to lengthen/shorten add/subtract surface area in an effort to improve flight characteristics. In actual practice, you'll find that if you show up at an IMAC meet with a model that is a fair resemblance of its full-scale version, you will not get any hassle from competitors or contest director. There is a point penalty for failure to have an instrument panel or pilot figure under the canopy, which becomes an issue when you get good enough to start winning trophies. So any of the various versions of Extras, Yaks, CAPs, Sukhois, Edges, Staudachers, Giles, etc. will do just fine. Bipes too, if that's your thing, but you don't see many of these at IMAC meets for whatever reason. Check out mini-iac.com for more details.

If you are building rather than ARFing, and specifically for IMAC, you have the advantage of making your control surfaces with less area than typically supplied with the popular ARFs, which seem to be all designed for 3D rather than precision aerobatics. I find it's a lot easier to set up control linkages with full servo resolution when I build rather than ARF. Or buy a Carden kit, which is more geared to precision aerobatics as distinguished from 3D.

ok thanks. so I will be ok with an Aeroworks extra 260

You will be just fine with that plane