I have been looking at the 3 rank Roman units and note that others are reporting that AFTER painting, the 3 rank combo is not making such a tight fit. I have put a base together on a dry fitting and they fit perfectly.
So I think 2 things are going on here. Firstly the primer and paint add a thin layer to make the dry fit tighter and secondly, the primer on the plastic is preventing the plastic cement from making a good plastic to plastic weld.
My approach will be, to dry fit and clamp the three ranks together between my fingers and then run 'brush on' primer over the top and around the sides, so that where each of the three ranks touch each other is coated, but the paint does not penetrate to the internal structure of the three rank system. Then separate and let dry, then prime the rest of the front and back of the unit - totally avoiding any innards, which after painting will then still be able to work with plastic cement.
I generally use a dark primer dry brushed with white. I am looking at using 6 colours after that to complete the unit.
Flesh - faces, hands, bare arms and bare legs
Red - shields, knee length tunic, use a second shade of red for tonal variation if you have to
Brass - helmet, back plate.
Mid Brown - wooden shaft to Pilum (spears), sandals, Hair if you have to, belt if you have to.
Lead - shield boss and rim, Pilum metal tips
White - feathers on the helmet.
Put a black wash, thinned to 50 / 50 over everything and make sure pooling is removed.
I will also try one base with a rattle can, just to see whether there is really any practical difference in the two approaches.