The rules are flexible for number of figures on bases, and, to some extent, number of bases that make up an element – I think there is a specific example of 3 15mm deep pike elements used as a unit
The original intention was square bases, but the author realized that many people were expanding DBA armies and made it flexible. Psiloi (skirmish) infantry are single DBA Bases
So I recommend creating only DBA legal bases in case you wish to expand your DBA army to big battles, or try out other rules. With friendly opponents, the most important thing is to be able to distinguish units – doesn't matter what is on the base
Bows are two DBA Bases (square element)
Javelin are two DBA Bases (square element)
Supported infantry one spear, one bow to create a square element
If your DBA Bases are 15mm deep, ADLG will still play well with 40x30 units, or you can create a sabot that adds 10mm or use 3 bases to create a 40x45 unit
John