The births website has it as "Solomon" but looking back at previous squad lists and retained lists - for both the PL and the EFL - he has been consistently shown as "Soloman".
I've never heard of such a requirement. In any case, without Rushworth we would have two - McGill and Steele. A quick check shows that a number of other clubs only have two goalkeepers listed, such as Aston Villa and Burnley.
This is the point though - the U-21 players always get listed by their "parent" club, even if they are out on loan elsewhere (and if they are on loan at another Premier League club, they'll be listed by that club too). For instance, Carl Rushworth, Jensen Weir and Marc Leonard were all out on...
It is a bit unusual to see him listed (for instance Steven Alzate, also out on loan, isn't listed). If he were to return in January he could play anyway (obviously clubs making signings in the January transfer window don't have to wait until the window has closed before they register and use...
A link, for those interested.
A few notes:
1) Albion name a squad of 20 (plus, of course, the 62 U-21s)
2) Carl Rushworth is named
3) Joao Pedro qualifies as a HGP