Negative numbers with additive ElGamal
Now I noticed that no matter which generator I use, when I use power-mod such that g^(q-1) mod q the result always 1

Congratulations, you just rediscovered Fermat's Little Theorem which says that for all primes $p$ and all non-zero integers $a$ which are not multiples of $p$, it holds that $a^{p-1}\bmod p=1$.

So if I have the group order set to 8009 minus one can is 8008, because 1 + 8008 mod 8009 = 0.

Indeed, this is how it works for plain addition $\bmod 8009$, however it seems you're not actually doing that and are actually using lifted ElGamal, i.e. encrypting as $(g^k \bmod p,g^m\cdot y^k\bmod p)$ (because standard ElGamal only has a multiplicative homomorphism).
But at that point you're no longer adding $m+m'$ but you're instead adding $g^m\cdot g^{m'}=g^{m+m'}$ and as it turns out, these exponents don't operate in the group $\mathbb Z_p$ (i.e. $\bmod p$) but instead in the group $\mathbb Z_{\operatorname{ord}(g)}$, where $\operatorname{ord}(g)$ is the smallest non-zero integer $q$ such that $g^q\bmod p=1$. If you are using a safe prime $p$, i.e. a prime $p$ such that $(p-1)/2=q$ is also prime, then $\operatorname{ord}(g)$ can take exactly 4 values: $1,2,q,2q$ (by Lagrange's theorem and the multiplicative order being $p-1$ for a prime).