Except the belt colors and forms were already built in before Ryo bonded with any of the Taros.
And the cars were completely pointless, what with how they just appeared out of nowhere at random instances. Heck, the whole idea of the giant monster battles in Den-O were dumb and just seemed like a cheap way to sell Denliner.
We have no proof of that since Ryotaro never transformed into any of those forms before he got the corresponding Imagin, so it's totally possible that the system adapted to each of the Taros
The buttons for each form, with their respective colors, were already there though. It could be just a coincidence. Their colors coincidentally matched the colors of their preferred forms, but it wasn't necessary... Although then Sieg arrives and adds a 5th form without any extra button and the Imagins all show later that they have personal weapons similar to their Rider form weapons, showing that the forms are customized to each one of them. The set up of Den-O's belt was just a mess.
We have no proof of that since Ryotaro never transformed into any of those forms before he got the corresponding Imagin, so it's totally possible that the system adapted to each of the Taros
not really. they were used for different combat situations just like the forms(Isurugi used to fight an aquatic Gigandeath, Rekko for a strong one, Ikazuchi for one the fought at high altitude), and on the flipside of things, the DenLiner battles seemed like a cool and exciting way to make a climax for the arcs they were used in