This post is more so I don't accidently hurt anything just doing what sounds right to me (like last time when I bought 2 turtles because I thought one would be lonely, btw don't do that unless you have A lot of room. If one ends up being male and one female

)..
Hello I am setting up my new 29 gal aquarium. I currently have a 20 gal with a bunch of guppies in it that will be moving over to the 29 gal. I want to change from the blue and black gravel setup I've had (I really wish I never mixed those two bags of gravel together) and switch to either all black sand, gravel, or a mix of the two). I plan on getting RCS and I read they like dark colored substrates.
I will lightly plant this tank, in the future for now I just want to get it set up. I also don't want to go through a full cycle so is it ok to put some gravel in a cup or small bucket and place it in the new tank so I can easily pull it out later after the BB has spread around a bit?
EDIT: I forgot to subscribe to my own thread when I was posting to know when I get responses. Is it to late? I don't see a button now.
Normally I would just take the filter out of the 20 but it will still be in use and I don't think I should take my filter from the guppies. The 20 gallon will eventually become the sump for the 29 gallon I just need to figure out how I'm going to attach them to each other.
So to make a long story short:
Can I mix sand and gravel together?
Should I just choose 1 type of substrate and not mix them?
Will I have a problem in the future with plants rooting in the sand?