Looking at this guide and it mentions many races (such as the dhampir) bypass the 1st level arcane spell requirement of the prestige class. Is this the case & if possible can someone link me to the relevant rules source?
There was back and forth discussion over whether Spell-Like Abilities counted as the ability to cast arcane spells for certain prerequisites. As of writing this, the current consensus is that they do not.
This FAQ says the following:
Spell-Like Abilities, Casting, and Prerequisites: Does a creature with a spell-like ability count as being able to cast that spell for the purpose of prerequisites or requirements?
Only if the pre-requisite calls out the name of a spell explicitly. For instance, the Dimensional Agility feat (Ultimate Combat) has "ability to use the abundant step class feature or cast dimension door" as a prerequisite; a barghest has dimension door as a spell-like ability, so the barghest meets the "able to cast dimension door prerequisite for that feat. However, the barghest's dimension door would not meet requirements such as "Ability to cast 4th level spells" or "Ability to cast arcane spells".
This is an overturning of a previous ruling, so the guide is probably a little out of date.
Thanks :)