No it is not mandatory to have full member rank to join signature campaign. Atleast you need to have a Member rank. Although being a Full Member gives you a lot.of choice. If you want to participate in signature campaign first concentrate on ranking up by creating good topics on the forum.
Rank to be able to wear signature is Junior member but no hyper link. Signature campaigns that pay in Bitcoin don't take Junior members. Some bounties accept Junior member rank.
Bounties are not good choice because many scam projects run bounties and don't process payments for participants after all. Rewards from bounties usually low in value when you sell them on exchanges. Sometimes you can get luck but you should not hope for big money from bounties.
One more bad thing, when you join bounties, you won't be forced to make high quality posts. Bounties don't help you to increase your post quality and you can not rank up. Full member rank is the min rank to join good campaigns pay in Bitcoin.