I use 12ax7 and 5751 interchangeably all the time. It should be safe without any risk of damage to gear.
There is however, an issue with gain in that the 5751 has only 80% compared to the 12ax7.
In my experience this translates to maybe 1 or 2 db difference in sound? So depending on the overall gain in your system, it may or may not work. If you are currently right on the edge of having enough grunt and headroom, 5751 may not work. Try them and see if they work for you.
If pairs are used, you should try to match them.
