Yea you can mix the pre tubes any way you like.
If you change the power tubes though you will have to get the amp biased. Although if you don't know when the tubes were last replaced/biased, then I would suggest getting ALL your tubes tested, as 1 bad tube will fuck up your sound.
Give Jim Wills from blue Glow a call. 0417970318. I will personally recomend his work. Most times he can get any tube you want, usually cheaper than dougs tubes online. Plus he checks & tests them all before they go in your amp. If they are damaged or test up substandard before going in, you don't wear the cost or the hassle of chasing up the supplier.
He modded the bias on my 5150, replaced all the capacitors & got me some sweetass tubes. Also added a kettle style plug to the back
I ordered pre & power tubes for my PITTBULL last tues, they arrived thurs, He tested all my old tubes for me, fitted & biased it within 45mins on saturday, 15mins before we jammed. The guy is a champ.
He is running RS components now out in osbourne park.