Your chief concern is distinguishing the IMing exchanges from regular dialogue and making sure readers can follow who is “speaking” at any given moment. Look inside Lauren Myracle’s TTYL to see how you can lay out the IM lines and the character names during an IMing exchange; it’s very clear in that novel. As for the graphics in TTYL, skip those for now. They’ll make your manuscript “gimmicky” to editors and agents. Leave that for a seasoned book designer when your book is being readied for production. In your ms, your creativity should be limited to picking an IMing font that is different from your narrative font but complementary, and perhaps using dark gray, black, and, if necessary, a complementary shade of blue to distinguish each speaker. You want clarity, not distraction.
I had a similar issue, though not to the extent Anne described. I just used italics. Same font, same size, just italics. And I used ‘screen names’ before each entry. It looked okay to me.