You don't really want to expose your RSJ as it will more than likely be clad for fire protection, although it will probably only be 2 layers of plasterboard. However, it's quite overkill fixing into a steel beam for a stud wall.
The head fixing to a stud is for lateral support, the length of wall, height and number of openings will depend on how much you need to restrain. If it's a short run of say less than 5m and around 2.4m high with no openings then try to make the stud frame tight up to the underside of the plasterboard, when i say tight i mean to the extent you are hammering in the timber uprights. not so that they are noticeably bowing though. You could apply a pu adhesive between the head plate fixing and the plasterboard and fix with about 4 screws through the timber into the plasterboard, fatter screws are better for this No10's, wedge into place until set (the screws are not holding the timber, see below) before adding in the timber sole plate and uprights & noggins at centre height. The wall will be pretty stable.
Don't worry if the screws don't go through and feel like they are hitting the bottom of the steel, this is exactly what you are after, what you are creating is a pin hold that will give you some lateral fixing. the depth of screw you need is about 75mm, if you think the beam is lined with timber battens before plasterboard then choose a deeper screw. The PU adhesive is my preferred choice as it expands slightly whilst setting, zig-zag a bead all down the length. Use 8mm frame fixings to the top (about 300mm down from top) of the two uprights fixed to the walls. you may as well use 2 more to fix the rest of the stud to the wall. You get 10 in a pack, 4 spare for another wall.
If you are not confident with stitch nailing the timber frame can i suggest you screw fix them, drill a pilot hole at 45º through the stud into the head/sole plate and screw through.
