I am planning to add about 240 icons to explore full rco soon for my new project. So anything this can do to make that easier is cool.
Yes, these will all work as ps3dxt5 , although I think that is a little lossy so if -ps3rgba8888 was an option too that would be great. So have 2 options for what to do with icons not in database that are found in folder or something like that.
240 new icons, thats a few of them

And are you telling the XMB displays them fine in -ps3rgba8888 and -ps3dxt5 ?... so you can choose which GIM settings to use. That doesnt uses to happen btw... usually the XMB is restrictive with GIM settings
But in this case that XMB gives you some freedom in which GIM settings to use... i suggest to make some tests with -ps3rgba4444 and -ps3dxt3
The reason why i mention this is because i guess all your icons for main XMB are in whitescale (like the officials)... so the amount of different colors stored inside the image is very small (are just white tones), and the levels of alpha needed are very small too (with a few should be enought)
Im going to explain something im sure some of you already noticed it, but in general, this is something you should know
The GIM format allows all this settings, lets say... half of them was supported by the PSP... and the other half was implemented specifically for the PS3... but most of them are not used in official firmware (as far we found but maybe there are some old firmwares using them and we dont know)
-ps3rgba5650
-ps3rgba5551
-ps3rgba4444
-ps3rgba8888
-ps3index4
-ps3index8
-ps3index16
-ps3index32
-ps3dxt1
-ps3dxt3
-ps3dxt5
-ps3dxt1ext
-ps3dxt3ext
-ps3dxt5ext
The numbers that appears at right of the name (like the 8888 of -ps3rgba8888) indicates the amount of "bits per pixel", and are ordered "by channel"
So in -ps3rgba8888 is doing Red=8 bits, Green=8 bits, Blue=8 bits, Alpha=8 bits.... for a total of
32 bits per pixel
This is what the old windows called "true colors" and is lossless (if you convert a PNG to GIM and then you convert the same GIM to PNG... the PNG is identical to the original)
All the others rgba based options (-ps3rgba4444, -ps3rgba5551, -ps3rgba5650) uses
16 bits per pixel... so the size in bytes is reduced to half
*btw... keep in mind all the GIM files (or better said, most) are compressed in ZLIB individually before are "packed" into the RCO
In your case im suggesting to try with -ps3rgba4444 because you really need the alpha (the last 4 most at the right), but for the images we was messing around the other day for impose_plugin.rco (taken from firmware 0.90) we dont need the alpha... so we could use -ps3rgba5650 (the 0 most at the right indicates it doesnt supports alpha)
The indexed formats (-ps3index4, -ps3index8, -ps3index16, -ps3index32) uses a color palette to optimize the bytes size of the image... i guess this ones are going to be very small, also i guess the official images made with -rgba8888 can be replaced by indexed formats
All the DXT based formats are lossy... but the quality loss is not much notable in some cases (depends of the kind of image) -ps3dxt5 is "the king" of the DXT familly, but -ps3dxt3 is not bad... and dxt1 is only used if the image doesnt uses alpha
And then are the DXT "extended edge" formats (-ps3dxt1ext, -ps3dxt3ext, -ps3dxt5ext). I know the official sony codename is "extended edge" but i dont know what it does... all i can say is it doesnt seems to follow any standard... is one of that sony inventions