السلام عليكم و رحمة الله و بركاته
1 – Yes. There doesn't seem to be any restriction concerning this and I never read any book of fiqh saying you have to wait any specific time between marriages. So there is that. But whether it is wise to do that is another topic.. esp. if it is your first time getting married. One of the conditions of taking more than one wife is being fair between the two. Can you be fair? Ask yourself this question. If you can, then Alhamdulillah. If not, then best to marry only one.
2 – When it comes to delaying the mahr ("dowry") being given, then it is okay if both the husband & wife agree. So in this case, the nikah is valid. And sexual intercourse may take place. But "no mahr" is not an option. Mahr has to be given. Allah سبحانه و تعالى makes that clear: وآتوا النساء صدقاتهن نحلة فإن طبن لكم عن شيء منه نفسا فكلوه هنيئا مريئا (..give the women [upon marriage] their [bridal] gifts - Qur'an 4:4).