IIRC, slim PS2s start at V12 through V15; V7 would be a fat one.
1. That is a feature your modchip does not support. Basically your chip turns off after the first boot and wont authenticate further media afterwards.
2. ModBo are clones of Matrix Infinity, so you should put the elf you want to boot in mc0:\BOOT\BOOT.ELF (case sensitive) and then hold R1 at boot to load it.
PS2: SCPH-35001 v4 | Matrix Infinity (Green Dot) | Maxtor 60GB
PSTwo: SCPH-70001 v13 | DMS4 SE Pro | HDCombo | Maxtor 45GB