Yes, many were illegal immigrants, but there was alot of overlapping realities going on at the time;

The first Americans to migrate en masse did so with the permission, and at the benefit, of the Mexican government, primarily because Americans wanted to relocate, and it was easier to let them deal with the Comanche as oppossed to used federal funds and soldiers. There were land grands for settlements, okayed by the federal gov., and with empressarios overseeing the moves. The most famous was that of Stephen F. Austin. There were some illegal American immigrants into Texas while it was part of Mexico before the revolution - but they were few and far between, and most were outlaws or debtors.

Most of the Texicans did not own slaves, but the richer ones who would later lead the revolution did tend to own slaves. Slavery was abolished in all Mexican states, including Texas-Coahulla, in 1829, much to the dismay of the rich Texicans, who would later form the insergency that created the Republic of Texas. The rebelious Texicans would later encourage immigration by any means, and that included outlaws as well as families to counter the Comanche.

The revolution was originally about the non-violen efforts of the Texas state government to petition for the separation of Texas from Coahulla, which would have given Texas more power within the federal government, and thus the chance to save slavery via grandfathering. But after Antonio Lopez de Santa Anna declared himself dictator, and slaughtered unknown amounts of rebels in small revolutions throughout the states, the Texians [their new name] declared independence, seeing that they could not reason with Anna. [but could be okay with slavery, very american]

America, the nation, originally shunned the Republic like the plague, and would do so for almost a decade due to the slave issue and the free vs slave state debate that would bloom into the American civil war.