I am a casual collector and bought some top loaders. Is it alright if I put low value near mint cards in them? Also, how do I get them out so I can move them to newer top loaders when the time comes?
Yes.
And you can use one of those page marker tags (with the sticky section at the bottom to stick the page). Stick it to the sleeve then you can easily lift the card out of the toploader.
Yes you can do whatever you like. As already suggested above it’s a good idea to use a sticky tab on a sleeve, this makes it easy to pull out and risk any damage to the card from fingers/nails.
This is what I use for storing many cards as semi rigid holders are thinner than top loaders so you can have more per space. It is also convenient for submitting to be graded.
I guess it depends on what you define as ‘low value’. Personally, I put more rare NM cards in a toploader for storage or just in a binder. Bulk low value stuff goes into a box instead or a binder as well.