Under 21's rules?

Can player represent england under 21's but then play for the full walsh national team instead of the full english?

As far as I know this can happen - Freddie Eastwood played for England U-21's but Wales full national team.

I think they can.

Rabiu Ibrahim is playing for Nigeria U-21, can he play for Portugal after ?

Unless they declare for that nation.

According to SI once they have played a competitive U21 game they can no longer switch Nation:

Just to confirm that in FM 2009, players become committed to a nation once they play a senior game for that country or represent that country in a competitive U21 or U20 fixture. Players can play U21/U20 friendlies or any U19 games without losing their ability to play for their other nationalities.

Article 15

1) Any person holding the nationality of a country is eligible to play for the representative teams of the Association of that country. The Executive Committee shall decide on the conditions of eligibility for any Player who has not played international football in accordance with par. 2 below, and either aquires a new nationality or is eligible to play for the teams of more than one Association due to his nationality.

2) With the exception of the conditions specified in par. 3 and 4 below, any Player who has already represented one Association in a match (either in full or in part) in an official competition of any category or any type of football may not play an international match for a representative team of another Association.

3) If a Player has more than one nationality, or if a Player aquires a new nationality, or if a Player is eligible to play for several Associations' teams due to nationality, he may, up to his 21st birthday, request to change the Association for which he is eligible to play international matches to the Association of another country of which he holds nationality, subject to the following condiditions:

(a) He has not played a match (either in full or part) at "A" international level for his current Association, and if at the time of his first full or partial appearance in an international match in an official competition for his current Association, he already had the nationality of the Association's team for which he wishes to play.

(b) He is not permitted to play for his new Association in any competition in which he has already played for his previous Association. A Player may excercise this right only once.

4) If a Player who has been fielded by his Association in an international match in accordance with par. 2 above permanently loses the nationality of that country with his consent or against his will due to a decision by a government authority, he may request permission to play for another Association whose nationality he already has or has aquired.

5) Any Player who has the right to change Associations in accordance with par. 3 and 4 above shall submit a written, substantiated request to the FIFA General Secretariat. The Players' Status Committee shall decide on the request. The procedure will be in accordance with the Rules Governing the Procedures of the Players' Status Committee and the Dispute Resolution Chamber. Once the player has filed his request, he is not eligible to play for any Association team until his request have been processed.