Hi, Trihead. Welcome to the forum!
I'm not a male or very young anymore but I thought I'd respond anyway.
There are other guys here who can respond more specifically. I can say that I've had MG my entire life and it wasn't until my later 30's that I began having more trouble.
When is your SFEMG? Did they tell you to be off of the Mestinon beforehand? It can interfere with the test. If you can, don't have it 24 hours before it.
Twitching is one of the side effects of the drug. Have you tried a smaller dose to see if you still have those effects? Sometimes doctors start patients out with a smaller dose so that their body can get used to the drug. More is not always better with Mestinon. Sometimes it's the time between doses that needs to be changed too. See the PDF below.
MG is a tough disease to have. There's really no way to predict how you will do with it. There's a big learning curve with this disease. If they decide that you do have it, you'll need to ask as many questions as possible to learn to live well with it. For example, there are certain drugs that MGers can't take.
Did they do the MuSK Antibody test too or just the Acetylcholine Antibody one? Get a copy of any test results! A neuro here kept a positive AChR Antibody test from me and I didn't find out about it until 7 years later. Not kidding.
So, who's doing your SFEMG? The Univ. of MN or Mayo? You should be warm, well-hydrated, off of Mestinon and caffeine products and be fairly "weak." If you have MG and it's in the early stages, you may not test positive on the SFEMG. Event the antibody tests can be negative early on in this disease.
My advice? Take one thing at a time. There will be plenty of time later to think about "what ifs" and physical activity. You can't "push" MG or it will push back. If you get worse, don't hesitate to either call your neurologist (do you have one?) or go to the ER. Yes, MG can get that bad but it's manageable and hard to say who will get worse and who won't.
Keep asking questions. I do live in MN, BTW, and seriously do know all of the neurologists here.
If you need the "scoop" on any good ones, let me know. I hope the test goes well.
Annie