Sharding Mongodb while limiting the number of documents to be migrated
For your 7 collections on the primary shard:

Enter as administrator on the primary replica (if you shard your replica) and create your collection there by inserting 1 document or creating an index there. When you create a collection via the mongos - shard client - then the collection is started on a random shard; if you create it on the shard itself first, then you know it's on the one you want.  shard1$ mongo --port 27018 localhost/mydb --eval 'db.mycol.insert({firstdocument:'hello'})'
For the 25 collections: 

Standard sharding indeed as you mention. Be patient with the distribution. The first shards will start distributing after a certain amount of data, can be 100k documents if the documents are small.
For the big collection:

Have a look at shard-tags. So, you have to tag your collection on shard1 with let's say : {status:'in_use'}; shard2 is tagged {status:'freeBeer'}. Then depending on the status in the document, the shard is chosen.
For the disk issue, I think you gonna have to write a script. Get a warning when 60% of a disk is used, and another warning at 80% to take action. Then you'll need to upgrade your disk or redistribute the chunks. Certainly the 2TB collection could give you space via the tag, to send more to shard2. Or, add a shard3 ... what is actually the purpose of sharding, to grow horizontally.
Choose shardKeys well, practice configuration and setup on a small server, practice moving chunks, adding tags, ....
Have a look at the AllChunkInfo script of Adam Comerford for you scripts and testing : https://github.com/comerford/mongodb-scripts