Sorry I'm a bit lost. Throwing in team have called a two man line out. This is matched by the defending team. I won't now allow the receiver to join and make 3 man, without someone dropping out. The only way I'll allow the throwing in side to add a receiver is to have a 'full' line out i.e. 8 players.
If I've read this right, I believe it is incorrect. I suspect, however, that I must have read it wrong.
When both teams line up with 2+receiver, you are quite correct to prevent the receiver joining without someone else dropping back - but this is only accurate until the ball leaves the thrower's hands, when the receiver can join the line at will. I suspect you know this, but you then go on to say:
The only way I'll allow the throwing in side to add a receiver is to have a 'full' line out i.e. 8 players.
However you read this, it has to be incorrect. Firstly, a conventional "full" lineout is 7 players not 8 - the hooker throwing in and his fellow forwards. Whether they choose to field a receiver is optional, and doesn't affect the numbers in the conventional lineout. Of course, a "full" lineout is actually made up of the hooker throwing in and all his team mates, so it comprises 14 lineout players. But even if the conventional lineout is used, once it has formed it would be illegal to add a receiver, as he has inevitably delayed in approaching the line of touch:
[LAWS]19.8(d) When the ball is in touch, every player who approaches the line of touch is presumed to do so to form a lineout.
Players who approach the line of touch must do so without delay. Players of either team must not leave the lineout once they have taken up a position in the lineout until the lineout has ended.
Sanction: Free Kick on the 15-metre line[/LAWS]
But what is the situation for a 5-man or 4-man lineout? You imply that it is different than the "full" lineout, but in fact it is not. Anything you can do in a full lineout you can do in a reduced (or extended) lineout.