What makes you say it was foolish? In OTL the Spanish and the Portuguese had great successes in establishing colonies in Mesoamerica, (also some great failures, but risk is part of the colonising enterprise!).
If the Kingdom of Scotland had succeeded in settling in modern-day Panamera then there is significant chance that a stable colony could be formed. If a canal is still built through the region, then that would offer them significant profits.
As to whether the British Union would still occur, I think it would. By this time England and Scotland did share a Monarch afterall. Besides, so long as two states existed on mainland Britain, the threat of war could never really be ignored - Union allowed both nations to combine for the better interests of all involved, (as has been noted more contemporarily with the European Community and the Mesoamerican Union).
If the British controlled both the Panameran and the Souiz Canals, then the Empire would very likely be in a good position indeed, its possible that the Empire could even be around today!