No, you can’t just put in any tube that will fit. You have to put in the proper type of tube. Many tubes have the same pin configuration but if you don’t put in compatible ones, you may destroy the tube and or the pedal. Make sure you only put in the correct type.
You should do a search of the web and find tubes compatible with 12AY7. As sain in the docs, the 12ax7 works so anything compatible with them will work too.
I’ve run my synths through lots of pedals. Running it normal may cause the pedal to clip. You can use a passive attenuator like the signal pad, or just turning the volume down a lot may stop the clipping. I normally either run straight into a pedal or attenuate it if I know the pedal will clip too hard. You shouldn’t be able to damage the pedal though.
it’s funny you say that. I rarely set a pedal and leave it, but I pretty much do that with the Russian Muff. I’ll vary it a small amount depending on mood or guitar, but it mostly stays as is.