I use the standard ball head that came with it. You get 2 options when you buy it, just the legs or with a head. The head is designed to be small enough to fit between the legs when they are folded back on themselves. If you use a head that is too big, which I think would be most to be honest, the tripod doesn't fold up to its smallest size.
Just tried it with my 1D4 and 100-400 Mk1 on it. Seems like it is strong enough for that, not rock solid like a 5 series but for something so tiny I would not expect anything better. Obviously you need to be careful with the centre column extended but that is the same for anything really. I avoid using it when I can. Certainly it is very useable with a load that large.
Using the lens tripod foot means that the load is extremely well balanced so you will have no issue with sag unless you are at some pretty extreme angles.
Hope this assists