You don't need to post heat treat the chromo assuming it was not hardened in the first place. Normal aircraft tubing is not hardened.
I don't know what kind of recumbent you are planing, but one thing to consider is if there is anything penetrating the frame. Like on mine, the head tube goes right through the main beam, which requires a pretty huge hole.
You can weld other steel components onto chromo. As long as they do not require the strength that chromo has. An illustration would be the springer front end on a chopper. When these get raked out and low enough, the angle can be about 30 degrees from the horizontal, and a most effective means of providing suspension is to have just two 4130 tubes. If you consider that situation, a normal cold rolled tube of the same dimensions, would be just as flexible and provide the same ride, except it would not have the strength. So anything you are attatching to your frame that does not require great strength in the most efficient dimensions, can be made out of mild or CR as long as the loads are not going to be too great. So an obvious example would be a home made cable stop out of mild. You don't have to worry about it being too weak, and it will weld to the Chromo fine also. Though chromo is known to weld with a very nice bead, when the proper filler wire is used.