Yes, in fact its recommended that you have a lot let over. Remember this garrisons are fixed, your effectively tying them down to your planets. After that they cannot be moved. So anything left over will make up the entirety of your invasion force. So be sparing.
Swapped Valorum into my Representative position. Next up will be dividing the troops up. Feel free to divide them evenly tomorrow if I haven't gotten anything ready :/
Just to be clear, do I designate ALL of my forces to one planet or another, or can I have a few left over, floating freely?