The only way to solve this kind of problem is to communicate with the people who are giving you trouble. You should do this one-on-one, not during a session. Give them respect and demand it for yourself. A few pointers.
Assume they mean well… mostly.
A fair amount of these trespasses could be well-intentioned. These players are veterans of the site and veteran GMs. They probably see you as someone who would benefit from their “help.” So when they prompt other players for checks or quote the rules to you or check monster stats, they probably think they’re being helpful. This sense they have that they know better than you then feeds into the less well-meaning trespasses, like grumbling about XP, but the mindset you want when talking to them is that they mean well. Assuming positive intent on their part will temper your own manner and make it less likely that they get defensive.
Make it clear why it is a problem.
Their behavior has several effects on you and your game, and you should tell them in no uncertain terms what those effects are. First, it slows things down. Second, it interferes with your development and improvement as a GM. Third, it confuses other players as to who exactly is running the game. Fourth, it interrupts you and forces you to concentrate on something other than telling a good story and moving the game forward. It takes you out of the moment and puts you in the wrong mindset to run the game. They might not realize this, so explain it to them clearly. This is a good point to ask them to think about how they would react if players acted this way in one of their games.
Firmly assert your right to run your sessions the way you see fit.
The Pathfinder rulebook is clear–the GM is the final arbiter of all the rules and if a rule isn’t what’s best for the game, then the rule can be ignored, or fudged, or altered. On the fly. Whenever you, the GM, wants. Raising a rules question is fine. Arguing about it with the GM is not and it won’t be accepted.
Make this clear: monster stats are not set in stone. This has been true since the first Monster Manual in the 1970s.
As for monster stats, modifying monsters to throw players a curve is as old as role-playing games themselves. Tell them that your monsters will not always conform to the exact Bestiary stats and there is nothing in the rules that says they have to. They should not get hung up on what the monster’s stats are “supposed” to be. That’s meta-gaming anyway.
Recognize that they have something to offer.
Assuming you’d like to play with these guys again, you should recognize their experience and the fact that you value it. But you need to develop as a GM yourself and you cannot do that if they’re chiming in all the time. Recognize that they can help but then ask them to please refrain unless you ask for their help. Once in a while during play, throw them a bone: “Hey, Mike, you know the rules backward and forward, what’s the roll for identifying this item again?”
End on a note that is positive but firm.
You want to leave things on a positive note. You’re part of an online community and you don’t want enemies in it. Emphasize how much you enjoy playing with them, both as player and GM. Tell them that you value their experience and their time on the site. Tell them that you hope they can live with how things need to be during the sessions that you run because you would hate to lose them.