Just tested it in a creative world:
Yes the tinker's construct tank can hold normal fluids (used seed oil to test it). Tested the 5x5x4 tank size and it holds 400buckets. (max is 11x11xbuildheight if I understand the book correctly. So it can become huuuge with alot of space)
There is a little problem which you have to work around: The drains from tinker's seem to not accept pipes/conduits as input/output at the moment.
But I already found a nice lil workaround for this:
- Add one enderIO tank to a drain set to "push" the fluid into the tinker's tank
- Another enderIO tank set to "pull" from a drain
this way you can use the enderIO tanks as input/output for the tinker's tank and then can add normally your pipes to these two tanks. So kinda like a giant addon for the enderIO tank if you want to see it like that *smirks*
Oh and until the output-enderIO-tank is full you get alot of flickering of the fluid, cause it's put in and directly pulled out again. Once the pulling enderIO tank is full it's all fine.
[edit]
Tested an 11x11x11 tank: Holds 5'324 buckets of fluid