I am no expert, but I know enough to injure myself.
Take it FWIW...
Ip means plate current, that is the spec that gets matched in a minimal test. You can't believe what is written on the box unless you are familiar with the company and how they test. You definitely don't want to mix different tube types together. But running one amp with 35ma quads and another with 42 is fine and would work good. They will both sound good, there might be a slight difference in tone, but I doubt you could hear it. The important thing is both monos would be running as designed. Some perfectionists may say they should all be the same, but I don't agree. Octets are for big PP tube amps that use 4 on a side per channel and all must match.
Push pull topology still has a bias adjustment, some may have 1 screw per quad, some even have one per tube. It is definitely adjustable, how easy I don't know on your amp. You connect an ammeter and adjust the current with a screw, just as WEEZ says. It should be somewhat easy unless there is some fancy beautification going on where the screw should be... haha Once set, check it a few times in first 100 hours, then one in 6 months or 500 hours whatever your instincts tell you. After break in they shouldn't change much. Always use the mfg current setting, no matter what people say about EL34 sounds best at 50ma, etc. The designer has it all on the line when he chose the bias current so that it would be safe, easy on tubes, and sound good enough sell a lot more amps by word of mouth from happy customers! Someone told me 6550s should run at 50ma. I set it there on my new (used) amp, and it sounded like crap. When I jacked them up to 75ma as manual said, it came alive.
If there is only one bias adjustment per quad, you want the tubes to be closely matched so they will all operate at correct current. If the amp has multiple adjustment screws, you can use different Ip tubes, you just gotta check that manual for instructions. Those quads you got, if correct, are an awesome match (except for the wrong tube....) Using mismatched Ip tubes on opposite sides of push-pull will result in crossover distortion which you will easily be able to hear, so keep the quads together per amp.
Since you are a new tuber (CONGRATS!!!! You wil love it) here are some tips I learned the hard way.
If you shut if off for changing wires or a quick tweak make sure you let er cool down before you turn it back on, not doing so can cause arc in rectifiers which will shorten their life. Rolling rectifiers had a good effect on my Cary. I got some 60s vintage RCAs.
If something goes wrong, or starts to act funny, just shut it off first, think about it later. Check your fuses before you bring it for service. You can usually fix a tube amp yourself. Keep some spare fuses on hand, and a spare set of tubes is handy too, even if they are just crappy spares to get you through waiting for shipping on replacements. Supposedly tubes don't like fingerprints so handle with a rag or gloves. Lift them out of the sockets by the base if you can. Pushing them in from the top is OK. You will find that power conditioners have a different effect on tubes than on solid state, the ones I have tried sound bad. Plug the amps straight into the wall before you final judgement on their sound quality. If you mfg says it's OK, and you like very clear powerful sound instead of sweet syrup try a set of Svet Winged C 6550s. That did it for me. Lastly, don't listen to those tube rollers who say "try this tube, it is so awesome" haha
Rich