There is also Primo (first) for really little kids, pre verbal even) which is 2x Duplo and yes, can interlock with normal Lego bricks.

ETA: looks like it was unfortunately discontinued: https://en.wikipedia.org/wiki/Lego_Baby . This Lego was great for my kid to learn motor skills like orientation, insertion, and removal (along with things like wooden veggies that connected with velcro).