Bigger challenge is the connection outbound in IAH. Arriving at different gates - so verify you are sitting in the very front of aircraft.

In SJC - it is same aircraft so no chance of a problem.

On public transportation -
EWR to LGA
Option A - There is a bus from EWR to Port Authority and a bus from Port Authority to LGA. Run you about $25 each way - you have to be conscious of time of day due to traffic delays. Without traffic maybe 50-60 minutes
Option B - Take AirTrain > NJ Transit to New York Penn to LIRR to Woodside and take a cab to airport. Most expensive but fastest route. About 10+20+20+15 = 45
Option C - Take AirTrain > NJ Transit to NY Penn to E train to Roosevelt AVe to Q33 bus. Cheap but Q33 takes a while to arrive and slow to LGA even though a short distance to cover. At minimum hr 20 minutes
Option D - (If time is no factor but money is) - Take Go28 or 62 to Newark Penn (or closest stop Market) > Path train to WTC > E train to Roosevelt Ave and board Q33 to LGA. I think this ride will cost $5.85.
Option E - ETS Airport has direct shuttle to LGA from EWR for $32 each way.
You should obtain MetroCard for C & D for free transfer between subway and bus.
In AM or PM rush hour - B,C,D are better than A simply because of traffic at tunnels in cross-town Manhattan.