Its fine to install DSiWare, that's basically the whole reason hiya exists, the limit is 39 titles (including system titles and the cartridge) and 20MB
in the 00030004 folder, if you use
TMFH to install as system titles though then there is no size limit.
Not safely, there is a way but currently all apps that can write files to NAND
always cause it to corrupt as they don't write the FAT copies correctly. This usually doesn't immediately have negative consequences but makes Unlaunch unable to install and is just not a good thing to do. You can also restore a NAND backup but that's bad as it writes over the whole NAND and the DSi's NAND is very cheap and tends to die after just a few NAND backup restores so that's not a good idea either, also the existing tools don't error check like at all so it's very easy to mess up. Just use TWiLight or hiyaCFW if you want DSiWare.