Manfrotto 501 502 504 and 509 heads all use the same plates. They are not acra swiss compatible. But they are quite standard in the video industry. The same plates can be used for Sachtler FSB6 and FSB8. We usually get long plates with a stabilizer lock to the camera body.
With Sachtler FSB6T, FSB8T (every model with a T at the end) and DV10, they use a different mini plate. Balance is achieved by a sliding mechanism in the top of the fluid head.
For a tamron 156 or sigma 156, fsb8 will be overkill. FSB8 is capable of holding the 400/2.8 at any angle you stop at even with the counter balance spring set at only 4. For tamron 156 a manfrotto 502 will be enough. For the sigma 156s, the manfrotto 504 will be enough. The manfrottos are very value for money. Once you move up to sachtlers the cost jumps exponentially.
Yes, The sachtlers look bit overkill for me

The only consideration now is plate. I have arca-swiss plates on my camera/lens (156s, 200/4) and on my monopod/mini-tripods now, and wonder if its a good idea to put an arca-swiss clamp on the 504PLONG plate, or remove the arca-swiss plates and mount the lens directly on the plate for a more secure grip.