Is ok to use regular 209 shot shell primers instead of those marked for muzzle loading?

you can use regular primer.

the problem with the regular primers is that they are powerful enough, by themselves, to be able to push the bullet out. that can cause your load to become unseated, and affect accuracy.

I've been seeing a few posts on the variflame(?) setup, and I'm considering moving to that myself.

Yes, they are cheaper and better. See this:

http://www.hpmuzzleloading.com/Technical3.html

Chap Gleason

A lot of people used to shoot the primers made for .410 shotshell, until all the muzzleloading primers came out. If you shoot the standard shotgun primers and your accuracy is inconsistant, that might be your problem with the rifle.

I shoot a Winchester W209 and Remington Kleenbore Primers.

Tried the Winchester 777 primers and did not like them. They left a worse crud ring with 777 powder than 209 primers did. Went back to the Winchester 209 primer: They are a lot cheaper too.

The worst primer I ever shot was a Winchester WML 209 not to be confused with the Winchester W209. On the outside of the Winchester WML 209 it claimed they were made for inline muzzleloaders. Those WML209 blew the breech plug retaining spring apart on my Black Diamond. On my Knight Disc, it started to wear out the back of the breech plug where the disc primer carriers rest against the breech plug. So we do not use them anymore.

i dont buy into the " its to powerful/Its to hot" therory. Ive never had anything like that happen and the accuracy was amazing. What i think it is, Put a little scare into the inline shooters and sell them a package of primers that costs $3-4 more than the regulars and claim they solve all these "problems".. I belive i've shot over 400 of the federal and cci shotgun primers and they were excellent performing primers that offered me less blowback.
To answer your question, Yes, You can use them.

I bought Remingtons to try with mine and then Saturday bought some Winchesters. Just firing the primer, the Rems seem dirtier inside the barrel but clean on the back of the breech and the firing pin. The Wins seem to leave more crud outside the back of the breech plug and around the firing pin but are cleaner inside the barrel. I also noticed last night while firing a couple to make sure the barrel was dry that the Wins don't seem to throw out sparks while the Rems do. The accuracy doesn't seem affected by either. The pyrodex fires and the bullet comes out the end, I can handle a little extra clean up I guess.

Some of us have different ideas on this subject. By going to a tight sabot I believe that it will hold its place long enough that the powder burns correctly, when you can shoot a considerable number of different bullets of several brands consistantly into groups under 2 inches at 100 yds and the best of the lot will go into a group less than an inch some times when conditions and your shooting are both good then there just can not be to much wrong!
Lee