Can't do HTV unless it is a vector graphic. Most people have no clue about those. What happens most often is you get a raster image and have to redraw it. Few can work Illustrator, CorelDraw or have even opened Inkscape.
You could insist on them only giving you vector. Most just can't. A compromise might be an additional charge to rework raster into vector, otherwise you will just have to bite the time needed to redraw them yourself, or work it into the final price.
HTV is far easier when doing one color. Each color adds to the complexity of the transfer. Setting a max color limit might be a good idea. Maybe providing a list of HTV colors you have would be good. Some designs are near impossible to get to go onto a shirt with multiple colors no matter what. HTV will shrink some when it hits the hit press. It's a thing you learn with experience when this is likely to happen, but things like outlines that press up against another color might not do well. You have to take this into your design consideration.
In the end, it might be best to produce the designs yourself. Few people know anything about vector graphics, besides the people that rely on them. If you are making the design, then you will know its viable. You can take design suggestions from customers, but as it will have to always be a vector graphics, and you have to restrict the color count and make designs that can actual be transfered, you would have to send them back a design for approval, that might be close to theirs, or might be far off. If it is far off, it might be a better design or a worse one in their minds. It is almost certainly going to be a more practical one.
