@PrintDriver gave you good advice.
A rule of thumb is to choose images that are at 300 ppi resolution for the final collage. Upsampling or enlarging a blurry image to a higher resolution will just result in a higher-resolution blurry image. If the original resolution is higher than 300ppi, that’s fine — it can be sampled down to 300 ppi.
3000x3000 pixels will get you a 10"x10" cover at 300 ppi. However, if the printing goes all the way to the edges, you’ll need an extra 1/8-inch bleed all the way around.
The 300ppi thing, like I mentioned, is a rule of thumb, which allows last-minute adjustments to size. If need be, a resolution of around 250ppi is totally adequate for most commercial printing. If these won’t be printed commercially, and if this cover is just a one-off thing for a friend that will be printed on an office desktop printer, a resolution of 150ppi is probably acceptable considering the quality of most desktop printers.
Smart objects have nothing to do with resolution. If you scale them up, their resolution will still decrease. Smart objects are useful, but not for this unless they’re vector objects that you’ll resize and subsequently flatten into raster objects.
300ppi is more than enough to print on a hoodie, but printing full-color images on sweatshirts comes with its own set of drawbacks (a different and lengthy subject). Solid, spot colors work much better for this.
As for a poster, the 300 ppi version for the album cover will likely be adequate, even if the poster is larger and the resolution subsequently decreases because of it. Higher resolution is needed for typical reading distance of about 18 inches, but posters are generally made for walls and will be viewed from greater distances that don’t require razor-sharp printing. Then again, much depends on the size of this poster and where it might hang.
Placing any typography or solid graphics into the collage is best done in a Vector application when possible since vector objects have no inherent resolution and will scale to any size.
Finally, if this is for a commercial product or something that will be used beyond personal use, using random images pulled off the internet could get you into trouble. Most images belong to their original creators and are illegal to use without their permission. There are lots of free-to-use or low-cost images online, but you need to know where to find them and whether or not they really are OK to use.